A study of zebra finches found them practising their songs. Other birds need to stay more alert, so only one side of their brain goes to sleep at a time

This is the month of sunsets like bonfires, blazing up over the islands in luridly gilded flames or sinking redly and smokily into a frieze of purple storm clouds. As the last bar of crimson neon fades from the channel through the strand, a final chink-chink from a shadowy blackbird brings the day’s proceedings to a close. City blackbirds have been found to wake half an hour ahead of those in the country, and to stay active six minutes longer (all this in Munich, for a start), but here on an ocean hillside they enjoy sunset to the final glimmer.

The blackbird sleeps deep in a bush, its claws safely locked upon a twig – sleeps . . . perchance to dream?

Birds share not only sleep with mammals but spells of REM sleep, too – dreams betrayed by rapid eye movement and distinctive excitations of the brain.

What might a bird dream about?

A study of zebra finches found them practising their songs. The researchers recorded the pattern of neurons firing in the brain’s musical area as the birds sang in daytime, then played the song back to them as they slept. Their brains responded with identical patterns, while still sleeping and making no sound.

The zebra finches were Australian but were studied by neurobiologists from Chicago University, long a centre for the study of sleep and its deprivation. Once, long ago, I visited the laboratory of Dr William Dement, who now, at Stanford University, is a leading authority on sleep. He was studying the loss of REM sleep in cats, and I remember his subjects regarding me, somewhat woozily, from beneath the Frankenstein electrodes implanted in their skulls. Today such implants are internal, microscopic and able to monitor the electrical activity of individual brain cells.

A study of European blackbirds, in Poland this time, showed that much of their EEG pattern echoed that of mammals, while showing the behaviour common to most birds: opening one eye as they sleep,

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In the state called unihemispheric sleep, one half of the bird’s brain is awake and alert while the opposite eye is taking a peek for predators. In the sleeping Polish blackbirds, the typical peek lasted from two to four seconds and accounted for perhaps 200 seconds in a night. This left plenty of time for REM sleep, but whether blackbirds singing in the dead of night are actually voicing their dreams aloud has not, it seems, been discovered.

Birds that sleep on the ground need to be especially watchful. Ducks and swans may have their heads turned backwards, bill tucked into the warmer air beneath their feathers, but they can still lift an eyelid. A video study of mallard ducks roosting in a group found that the birds sleeping at the edge spent longer with one eye open, looking outwards for threats, than those in the safer middle.

With a nice sense of mammalian priority, the lead researcher, Niels Rattenborg of the Max Planck Institute for Ornithology, in Germany, also recently led a team to study the sleep patterns of brown-throated three-toed sloths in a Panamanian rain forest. This pioneered the use of lightweight EEG recorders in wild animals, but it revealed in the process, not quite amazingly, that sloths who need to keep an eye out for predators sleep some six hours less in a day than do sloths in captivity.

The ability of birds to sleep with one eye and half a brain open – “micro-naps” – may explain the ability of millions of day-living songbirds to migrate long distances at night without arriving exhausted, weak and vulnerable.

American gulls have been seen flying to their roosts with only one eye open, and swifts have long been shown to spend all their nonbreeding time in the air, able to sleep on the wing. All this is grist to the work of neurobiologists chasing, almost incidentally among sleep’s secrets, the survival value of dreams.

The ability of humans to tell one bird from another might seem to be well served by the number of field guides in print, but their painted illustrations, however accomplished, don’t always suit the video age. The 1,600 or so photographs that throng The Birds of Ireland (Collins Press, €14.99) may not actually move, but they offer a vivid and extra avian presence.

Nearly all were taken by Mark Carmody, whose impressive galleries have enriched previous books on birds of the shore and lakes. Here he matches text by Jim Wilson with as many as 15 photographs per species, showing them in different seasonal plumages and postures, and from different angles – even how they look as a flock in the air. Similar birds are grouped together, so that the waders and warblers, in particular, are more easily told apart. This is the first guide of its kind for this island and should recruit yet more followers to the fast-flying world of BirdWatch Ireland.

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